1. Field of the Invention
The present invention relates to a control device for a multi-cylinder internal combustion engine that is equipped with a variable compression ratio mechanism.
2. Description of Related Art
A multi-cylinder internal combustion engine that is equipped with a variable compression ratio mechanism which moves the cylinder block relative to the crankcase is known. In such an internal combustion engine, a target mechanical compression ratio is set based, for example, on the engine load to control the variable compression ratio mechanism so that a desired mechanical compression ratio ((combustion chamber volume at the compression top dead center+piston displacement)/combustion chamber volume) can be simultaneously achieved in all the cylinders.
However, even if the variable compression ratio mechanism is controlled as described above, the actual mechanical compression ratios in the cylinders may not be necessarily uniform and the power outputs from the cylinders cannot be uniform unless the combustion chamber volume at the compression top dead center is the same in all the cylinders. Thus, an idea for providing a building-up portion on the top of the piston or on an inner surface of the cylinder head of each of the cylinders and cutting the building-up portion in each of the cylinders in assembling the internal combustion engine so that the combustion chamber volume at the compression top dead center can be the same in all the cylinders when the variable compression ratio mechanism is controlled by a specific amount is proposed (refer to Japanese Patent Application Publication No. 2009-008016 (JP-A-2009-008016)).
Even if the combustion chamber volume at the compression top dead center is made uniform for all the cylinders as described above when the variable compression ratio mechanism is controlled by a specific amount, the actual combustion chamber volume at the compression top dead center may not be the same in all the cylinders and the actual mechanical compression ratios in the cylinders cannot be uniform if the cylinder block is slightly inclined and moved relative to the crankcase when the target mechanical compression ratio is changed to control the variable compression ratio mechanism, for example. Therefore, it is desired to determine whether or not the actual mechanical compression ratios in the cylinders are uniform so that measures can be taken if necessary.